Added support for row table transformations

When testing single things with large numbers of values it's often handy to lay them out as a row hash table, rather than the usual column hash table this patch adds support for allowing transformations on those tables.

This should allow us to make these steps more re-usable like we already can for column based tables

Here's a little example:

The PHP for the context
```php
          /** @Transform rowtable:username,age */
          public function createUserFromRowTable(TableNode $table) {
              $hash     = $table->getRowsHash();
              $username = $hash['username'];
              $age      = $hash['age'];

              return new User($username, $age);
          }
```

Example input Gherkin
```gherkin
        Scenario:
          Given I am user:
            | username | ever.zet |
            | age      | 22       |
          Then Username must be "ever.zet"
          And Age must be 22
```

Depends on: Behat/Gherkin#89
``mixed`` is enough. It already covers ``null``.